1. Field of the Invention
This invention relates to a method for recovering lithium chloride used as a polymerization assistant in the preparation of polyarylene sulfides.
2. Description of the Related Art
Polyarylene sulfides prepared in are general by reacting a dihalogen aromatic compound with an alkali metal sulfide and/or an alkali metal hydrosulfide in the presence of a polar solvent such as N-methyl-2-pyrrolidone (NMP). Lithium chloride is used in this case as a polymerization assistant for whiting, reducing the salt content and controlling the molecular weight of PAS.
Lithium chloride as a polymerization assistant need to be recovered after the reaction for re-use because it is used in a larger quantity almost equal to that of the alkali metal sulfide and/or the alkali metal hydrosulfide, and is expensive. For this reason, there has hitherto been proposed a method for recovering lithium chloride from a reaction mixture from the preparation process of PAS. For example, there have been known a method comprising removing a solvent from the reaction mixture, washing the reaction mixture with an acidic aqueous solution, blowing carbon dioxide into or adding carbonate to the washing liquid to precipitate lithium carbonate for recovery as lithium carbonate, and a method comprising recovering lithium chloride in the reaction mixture on calcination.
However, the former method of recovering lithium chloride as lithium carbonate is disadvantageous in that (i) equipment and operation for recovery tend to be complicated, (ii) a larger quantity of auxiliary materials for conversion into lithium carbonate is necessitated, (iii) lithium chloride cannot be recovered directly and (iv) impurities and oligomers are left in the polymer to deteriorate the quality of PAS.
The latter calcination method is also disadvantageous in that (i) calcination conditions need to be selected meticulously to prevent lithium chloride from being volatilized off during calcination or to prevent loss of the polar solvent, (ii) the product quality is lowered due to excessive calcination residues, (iii) recovered lithium chloride has a poor color tint; and (iv) since SO.sub.2 is produced due to higher temperatures during calcination, it becomes necessary to use special materials for a calcination furnace.